Magnetic resonance imaging screening method

ABSTRACT

A method to screen individuals specifically for paramagnetic or ferromagnetic objects they may be carrying or wearing, before they enter the high-field region of an MRI suite. The device used comprises either a screening portal or a compact, hand-held magnetic gradiometer and its electronics. The method places all of the sensor arrays in close proximity to all parts of a subject&#39;s body, for screening purposes.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of copending U.S. patent applicationSer. No. 10/681,033, filed Oct. 7, 2003, for “Magnetic Resonance ImagingScreening Method and Apparatus”. U.S. patent application Ser. No.10/681,033 relies upon U.S. Provisional Pat. App. No. 60/440,697, filedJan. 17, 2003, for “Method and Apparatus to Use Magnetic EntrywayDetectors for Pre-MRI Screening”, and U.S. Provisional Pat. App. No.60/489,250, filed Jul. 22, 2003, for “Ferromagnetic Wand Method andApparatus for Magnetic Resonance Imaging Screening”.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is in the field of methods and apparatus used toprevent the presence of paramagnetic or ferromagnetic objects near amagnetic resonance imaging (MRI) system.

2. Background Art

Paramagnetic and ferromagnetic objects are highly unsafe near MRIsystems, because the strong magnetic gradients caused by MRI magnetsexert a strong force on such objects, potentially turning them intodangerous missiles. Several accidents, some fatal, are known to haveoccurred as the result of someone inadvertently carrying such an objectinto the MRI room. Current MRI safety practices rely on signage andtraining to prevent people from taking such objects into the MRIchamber. There is currently no known technical means in use to preventthe accidental transportation of such objects into the MRI chamber, oreven to warn of such an occurrence.

Use of conventional metal detectors, whether portals or wands, would notbe efficient for this purpose, because they do not distinguish betweenmagnetic and non-magnetic objects, and only magnetic objects aredangerous. Conventional systems generate an audio-band oscillating orpulsed magnetic field with which they illuminate the subject. Thetime-varying field induces electrical eddy currents in metallic objects.It is these eddy currents which are detected by the system, to revealthe presence of the metallic objects. There is no discrimination betweenferromagnetic objects, which are dangerous near an MRI system, andnon-magnetic objects, which are not. As a result, conventional systemswould generate far too many false alarms to be usable in thisapplication. The invention described herein solves the problem bydetecting only paramagnetic and ferromagnetic objects, which are exactlythose that must be excluded from the MRI room.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a method for scanning a patient orattendant for the presence of an object which is either permanentlymagnetic or susceptible to being magnetized by an external field. Thesensors used in this scanning method can be mounted on either a wandtype frame, or a portal type frame. Either embodiment positions theentire sensor array in proximity to every portion of a patient or otherindividual. The wand embodiment of the scanner can be passed inproximity to every portion of the subject's body. The portal embodimentof the scanner arranges the sensors in a horizontal alignment, makingthe sensor array suitable for positioning every sensor in proximity tothe body of a recumbent patient, as the patient passes through theportal.

The sensors can detect the magnetic field of the object, whether theobject is a permanent magnet or merely susceptible to magnetization.Where an external field induces a magnetic field in the object, theexternal field may be the Earth's magnetic field, or it may be generatedby another source, such as a nearby MRI apparatus or a dedicated sourcesuch as one mounted on the frame of the apparatus.

The novel features of this invention, as well as the invention itself,will be best understood from the attached drawings, taken along with thefollowing description, in which similar reference characters refer tosimilar parts, and in which:

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic showing the horizontal arrangement of sensorarrays in a first portal type embodiment;

FIG. 2 is a schematic of a second portal embodiment;

FIG. 3 is a schematic of a third portal embodiment;

FIG. 4 is a schematic of a first wand embodiment;

FIG. 5 is a schematic of a second wand embodiment;

FIG. 6 is a schematic of a third wand embodiment; and

FIGS. 7 through 10 are schematics of several embodiments of thearrangement of the source fields and sensors.

DETAILED DESCRIPTION OF THE INVENTION

The present invention, which applies to both permanently magneticobjects called “hard” ferromagnets and non-permanent magneticallysusceptible objects called “soft” ferromagnets, can use magnetometerswith good sensitivity at frequencies all the way, or nearly, to DC,i.e., zero frequency. This allows several modes of use:

(1) As a completely passive system, the present invention detectsferromagnetic objects using their permanent magnetization, in the caseof “hard” ferromagnets, or the magnetization induced by the Earth'smagnetic field, in the case of “soft” ferromagnets.

(2) As a DC magnetic susceptometer, the present invention applies astatic DC magnetic field, allowing control and usually enhancement ofthe magnetization of soft ferromagnets, thus enhancing theirdetectability.

(3) As an AC magnetic susceptometer, the present invention applies anoscillating AC magnetic field, but at very low frequencies compared toconventional detectors, allowing enhancement of their magnetization. Thepurpose of AC illumination is to move the signal from DC to a region oflower noise at finite frequency. The AC frequency is chosen to avoidinducing the electrical eddy currents detected by other systems, tosuppress the response from non-ferromagnetic metal objects, and thusmaintaining the discrimination capability.

The present invention importantly arranges an array of sensors in such away that the entire sensor array can be placed in proximity to allportions of the body of a subject, such as a patient or an attendant. Inparticular, the sensor arrays are arranged so as to be susceptible toplacement in proximity to all portions of the body of a patient lyingrecumbent, as on a stretcher or gurney. This object is accomplished byeither of two major embodiments of the invention: a portal structure,and a hand held wand. The portal structure is designed to have one ormore horizontally arranged sensor arrays, suitable for alignment of theentire sensor array with a recumbent patient. This differs from a portalarrangement in which the sensor arrays are arranged vertically, placingonly a few of the sensors in proximity to a recumbent patient. The wandis susceptible to movement over the body of the subject in order toplace the entire sensor array in proximity to all portions of thesubject's body.

A passive magnetic embodiment of the portal used in one embodiment ofthe present invention can be similar in some respects to the SecureScan2000™ weapons detection portal which is manufactured by QuantumMagnetics, Inc., and marketed by Milestone Technology, Inc., or thei-Portal™ weapons detection portal which is marketed by QuantumMagnetics, Inc. In important respects, however, the portal would bemodified to be suitable for use in the present invention, namely, tomake it suitable for use with a recumbent subject lying on a gurney orstretcher, rather than walking upright. In the known configuration,patients on gurneys would be too distant from too many of the sensorsfor adequate detection.

The portal includes two panels of sensors on the sides of the entryway.An array of magnetometers inside each panel enables detection,characterization, and localization of ferromagnetic objects from thesoles of the feet to the top of the head. The magnetometer array cantake a variety of configurations, and it can use a variety of sensortechnologies. For example, a set of 16 single-axis magnetic gradiometerscan be arranged with 8 in each panel. Other configurations can includearrays of multi-axis gradiometers, or combinations of single-axis andmulti-axis gradiometers. One or more magnetic tensor gradiometers mayalso be used. A magnetoresistive magnetometer, or any other sensorcapable of sensing magnetic field changes at or near zero frequency, canbe used.

As shown in FIG. 1, in order to scan a patient on a gurney, the portalsensor configuration 10 of the present invention must be arranged tobring all of the sensors closer to the patient and to effectively scan apatient in the recumbent position. Rather than being arranged verticallyas in the aforementioned known portals, the two sensor panels 12, 14 canbe arranged horizontally, parallel to the path of the gurney and oneither side, as shown in FIG. 1. This places the sensors in a similarrelation to the patient as they would have, in the vertical arrangement,to an ambulatory patient. Also, a single “snapshot” of data covers theentire gurney and patient, as in the ambulatory case. The sensor panels12, 14 can be permanently arranged horizontally, or they can pivot tothis configuration.

Alternatively, in addition to the vertically arranged sensor panels asin the aforementioned known portals, the portal can have a “dutch door”with an additional, horizontal, sensor panel 16 in the upper half of thedoor, just high enough to clear a patient on a gurney, as shown in FIG.2. As the patient is wheeled under the upper door, the patient wouldpass in close proximity to the horizontal sensor panel 16, allowing allof its sensors to scan the patient from head to foot, or vice versa.This gives the best detection and resolution of objects, since moresensors are placed closer to the patient. Then, the attendant would pushthe dutch door open and walk through the portal, being scanned by thevertically arranged sensor panels. The “dutch door” array 16 can bespring loaded, so that it moves out of the way for an ambulatorysubject. A microswitch indicator can tell the software whether the dooris engaged, for a recumbent patient, or disengaged, for an ambulatorysubject. As a variation of this embodiment, a portal with verticallyarranged sensor panels can be situated next to a portal with ahorizontally arranged sensor panel, as shown in FIG. 3.

As an alternative to the passive magnetic portal, an AC or DCmagnetizing field can be provided by one or more source coils, a DCfield can be provided by a permanent magnet array, or a DC field can beprovided in the form of the fringing field of a nearby MRI magnet. Inany case, a computer is provided to interrogate the sensors and tointerpret the magnetic signals, to detect, characterize, and locateferromagnetic objects. Characterization of the object provides the sizeand orientation of its magnetic moment, which can be related to thephysical size of the object, and to the magnitude of the attractivemagnetic force. The analysis software can use various known algorithms,or a neural network can be used. The information gained can be relatedto a photographic image of the subject, for the purpose of locating theferromagnetic object on the subject. A light display can be used toindicate the approximate location of the detected object. Systemdiagnosis, monitoring, and signal interpretation can be done via theInternet, if desired.

As an alternative to the portal type screening apparatus, a hand-helddevice can be used to screen individuals specifically for stronglyparamagnetic or ferromagnetic objects they may be carrying or wearing,before they enter the high-field region of an MRI suite. In someinstances, the lack of floor space precludes a fixed installation suchas the portal disclosed above. In these cases, a hand wand may be thepreferred embodiment.

The hand-held device, or wand, comprises a compact magnetic gradiometerand its electronics. The gradiometer can measure either a singlegradient component, multiple components, or the complete gradienttensor. The gradiometer comprises one or more pairs of magnetic sensorsand reads out the difference signal between members of each pair.Background fields have small gradients, so the difference signalresulting from these is small. Close to a paramagnetic or ferromagneticbody, however, field gradients are strong; they vary as 1/r⁴ with thedistance r from the sensor to the magnetic body. A strong anomaly issensed whenever the wand is passed close by such an object of interest.The wand does not detect nonmagnetic metals. Its electronics read thesignals out and process them. The output can be in the form of a simplealarm when the signal exceeds a threshold. More robust processingalgorithms can incorporate adaptive background cancellation to furthersuppress background gradient interference, and target-objectlocalization in the case of full tensor gradiometer implementations.

To increase the signal from the target object, it can be desirable tomake the measurement in a stronger ambient field than the earth'smagnetic field, which is about 0.5 Gauss. The fringing field from amagnetic resonance imaging (MRI) magnet can provide such an enhancedfield, with strengths in excess of 10 Gauss.

A further embodiment combines the magnetic wand with a wire coil thatcan be used, by means of driving electric current through it, togenerate a controlled source field. The coil can be configured tosuppress its common-mode signal on the gradiometer sensors but provide amagnetizing field around the wand. This field, by magnetizingparamagnetic or ferromagnetic objects, increases their signal relativeto the background. The field can be static (DC) or time-varying (AC).The benefit of an AC field is that the system can work at a non-zerofrequency, further suppressing background interference. The frequency ischosen to be low enough, however, not to excite a response fromconductive but nonmagnetic objects.

This device consists of a rigid, non-metallic, non-magnetic structurethat supports one or more pairs of magnetometers. Each pair consists ofsensors aligned to measure the same component of the magnetic field.Each pair's two outputs are differenced to create the gradient signal.Sensor electronics operate the sensors and perform the differencing.They also operate signal processing algorithms to suppress backgroundinterference and to alarm in the proximity of paramagnetic orferromagnetic objects.

In embodiments involving an active magnetic source, the wand also hasone or more coils of wire and electronics to drive controlled currentsin the coils, to act as a magnetizing source field. The coils aredesigned to produce a zero differential signal on the gradiometers, inthe absence of nearby magnetic objects.

In a further embodiment, an applied DC magnetic field can be created bymeans of one or more permanent magnets mounted in the wand. The magnetsare mounted such that their primary magnetic field is orientedorthogonally to the sensitive axis of the magnetometers in the wand. Inthis way, the sensors are not saturated by the applied DC field, butremain sensitive to enhanced magnetization of a ferromagnetic object bythat field. Use of permanent magnets to generate the field has anadvantage over using a coil, namely, the permanent magnet draws nopower. However, a potential disadvantage is that the magnetic fieldcannot be turned off, so the wand must be stored carefully when not inuse.

The use of AC fields enables the use of induction coil sensors, inaddition to or instead of magnetometers, like magnetoresistive,fluxgate, and other types. Induction coil sensors are impossible to usein the DC embodiment because the induction coil has zero sensitivity atzero frequency. Using induction coil sensors typically reduces the costof the product without sacrificing sensitivity in the AC system. Usinginduction coil sensors confers a particular advantage, in that itrenders the wand insensitive to interference from noise induced by thewand's motion in the Earth's field. This is a major potential source ofinterference in the case of the DC applied field.

An AC system could make use of two different excitationdirections—operating at two different frequencies, to avoidcrosstalk—which can improve detection of long, narrow objects, which areprecisely the shape that is most dangerous in this situation.

The wand can be extended into a two-dimensional array of sensors toenable reliable scanning without as much moving of the wand back andforth. Too large an array becomes unwieldy and expensive; the optimumarray size depends upon the balance between cost, reliability, and userskill found in any given application.

FIGS. 4 and 5 illustrate the principles of the wand embodiments 20 ofthe invention, utilizing an AC source. An excitation coil 22, by meansof a sinusoidal (AC) current driven in it, generates an alternatingmagnetic field that excites a combination of magnetization current andelectrical eddy current in any conductive and/or ferromagnetic and/ormagnetically permeable body nearby. The excitation frequency is chosento be low enough so that the magnetization (or, equivalently, magneticsusceptibility) response of objects to be detected exceeds their eddycurrent response. The choice of frequency remains to be determined, butit is expected to be several tens of hertz (Hz), or at leastsubstantially less than 1 kHz.

The excitation current can be driven by any number of standard drivecircuits, including either direct drive (controlled voltage source inseries with the coil) or a resonant drive (voltage source coupled to thecoil via a series capacitance whose value is chosen such that, incombination with the coil's self-inductance, the current is a maximum ata desired resonant frequency given by ½π(LC)^(1/2)).

In both FIGS. 4 and 5, the receiver or sensor coil is, in fact, made oftwo coils 24A and 24B, wound in opposite senses and connected in series.They form what is well-known as a gradiometer; a uniform magnetic fluxthreading both coils produces zero response. Coils 24A and 24B aredistributed symmetrically about the excitation coil 22 such that, in theabsence of any target object (which is conductive, magnetic ormagnetically permeable) nearby, each senses an identical flux from theexcitation which thus cancels out. A handle 28 can contain theelectronics and a battery.

Although the intent is to make the two coils 24A and 24B perfectlyidentical, and to place them in identically symmetric locations, inpractice one falls short of the ideal. As a result, any actualembodiment will display a nonzero response to the excitation, even inthe absence of a target; this residual common-mode signal is referred toas an “imbalance” signal. Standard electrical circuits can zero out theimbalance signal by adding an appropriately scaled fraction of thereference voltage V_(ref) (a voltage proportional to the excitationcurrent, obtained by measuring across a series monitor resistor) to theoutput voltage V_(out).

When a target object is near to either coil 24A or 24B, it spoils thesymmetry and thus induces a finite signal. This signal oscillates at thesame frequency as the excitation. Standard demodulation orphase-sensitive detection circuits, using V_(ref) as the phasereference, measure the magnitude of V_(out) in phase with V_(ref) and inquadrature (90 degrees out of phase) with V_(ref). At an appropriatelychosen low frequency, the response will be dominated by thesusceptibility response, which appears predominantly in the quadratureoutput, as opposed to the eddy current response, which appearspredominantly in the in-phase component.

In principle, the coils 24A and 24B could be replaced by twomagnetometer sensors (fluxgate, magnetoresistive, magnetoimpedance,etc.). Coils respond to the time derivative of the magnetic field, whilemagnetometers respond to the field itself; the coil's output voltage isshifted by 90 degrees with respect to a magnetometer's. If magnetometersare used instead of coils, then the susceptibility response would showup in the in-phase component and the eddy current response (at lowfrequency) in the quadrature component.

If the operating frequency is chosen much too high, both susceptibilityand eddy-current responses appear in the in-phase component (usingmagnetometers) or quadrature component (using coils), but with oppositesign, making it impossible to distinguish between the two. Atintermediate frequencies, the eddy current phase is intermediate betweenthe two components, complicating the distinction. Therefore, it isimportant to choose the excitation frequency to be low enough, andpreferably less than about 1000 Hz.

The substrate or coil form 26 must be nonconductive, nonferromagneticand, with one possible exception, magnetically impermeable (μ=μ_(o),where μ_(o) is the permeability of free space). The exception is that amagnetically permeable core inside the sense coils 24A, 24B (practicalonly in the cylindrical geometry of FIG. 4) can increase the sensitivityof the system.

Using a resonant drive circuit for the excitation coil 22 maysignificantly reduce the electrical power needed to create theexcitation. Thus, this embodiment may be preferred for abattery-operated, hand-held wand. The other circuits, includingdemodulation, threshold, discrimination, and alarm/alert, requirenegligible power, so the system power is dominated by the excitationrequirement.

As shown in FIG. 6, the DC embodiment of the wand 30 can have a sensorboard with 2 sensors 34, which can be placed at each end of an epoxyfiberglass paddle 36. A DC magnetic field source 32, such as a permanentmagnet, an example of which is a ferrite disc, can be mounted in such amanner as to provide a normal (perpendicular) magnetic field at thesensor 34. The concept of this arrangement is to provide an externalmagnetic field source to induce magnetization in any local ferromagneticbody, so that the sensor 34 can detect that body, while, at the sametime providing no in-the-plane-of-the-sensor active-axis magnetic field.

The use of a reference sensor helps to eliminate common mode errorsignals. For instance, a nearby passenger conveyer, such as a gurney,could contain magnetic components, but this spurious magnetization isnot what is intended to detect, and, therefore, it is preferable toeliminate this magnetic source.

An audio alert 37, such as a buzzer, and/or an alarm light 39 can beemployed to signal the presence of an unwanted ferromagnetic object. Aferromagnetic bobby pin is an example of such an unwanted ferromagneticobject.

A non-ferromagnetic covering material, constructed, for instance, of asubstance such as aluminum or nylon, or other suitable material, cansurround the wand 30. This type of covering is not only protective; italso facilitates removal of any ferromagnetic objects which might stickto the wand.

As shown in FIG. 7, the sensor's sensitivity axis is orthogonal to theaxis of the magnetic field of the permanent magnet 32. Otherwise stated,the magnetic field of the permanent magnet 32 is normal to the plane ofthe sensor 34.

In FIG. 8, the magnetic field of the DC permanent magnet field source 32magnetizes the ferromagnetic object, which then has a magnetic field ofits own, as shown in FIG. 9. This induced magnetization (“demag field”)is detected by the sensor 34, triggering the alarm buzzer 37 and/orlight 39.

An alternative wand configuration, shown in FIG. 10, utilizes twopermanent magnets 32A, 32B, as the magnetic field between them is lessdivergent than with a single permanent magnet. With the use of twopermanent magnets 32A, 32B and less resultant divergence, there is lessneed for criticality about positioning the permanent magnet with respectto the sensor 34.

While the particular invention as herein shown and disclosed in detailis fully capable of obtaining the objects and providing the advantageshereinbefore stated, it is to be understood that this disclosure ismerely illustrative of the presently preferred embodiments of theinvention and that no limitations are intended other than as describedin the appended claims.

1. A method for excluding objects from proximity to an MRI instrument,said method comprising: providing an array of sensors adapted to detecta magnetic field of an object; positioning the entirety of said sensorarray to scan all portions of a human subject; processing signals fromsaid sensor array to detect said object.
 2. The method recited in claim1, wherein said object comprises a ferromagnetic object having aninduced magnetic field, said method further comprising orienting saidsensor array relative to a source of an external magnetic field todistinguish between said external magnetic field and said inducedmagnetic field of said ferromagnetic object.
 3. The method recited inclaim 1, further comprising: mounting said sensor array on a portalstructure; and passing a recumbent patient through said portal structureto accomplish said positioning of said sensor array to scan saidrecumbent human subject.
 4. The method recited in claim 3, furthercomprising: providing said sensor array as at least two sensorsub-arrays; mounting a first said sensor sub-array on a movablehorizontal member on said portal structure; mounting a second saidsensor sub-array on at least one vertical member on said portalstructure; passing said recumbent patient through said portal structure,beneath said horizontal member, to accomplish positioning of said firstsensor sub-array to scan said recumbent human subject; moving saidhorizontal member to clear a passageway through said portal structurefor an upright human subject, said passageway being adjacent to said atleast one vertical member; and passing said upright human subjectthrough said passageway to scan said upright human subject.
 5. Themethod recited in claim 1, further comprising: mounting said sensorarray on a hand-held frame; providing a source of an external magneticfield mounted to said hand-held frame; orienting said sensor arrayrelative to said hand-held frame to distinguish between said externalmagnetic field and an induced magnetic field of said object; and passingsaid hand-held frame over said human subject to accomplish saidpositioning of said entire sensor array to scan said human subject. 6.The method recited in claim 5, further comprising: providing anelectromagnet mounted to said hand-held frame as said external magneticfield source; and generating an electromagnetic external field with saidelectromagnet.
 7. The method recited in claim 6, further comprising:providing an AC electromagnet mounted to said hand-held frame; andgenerating said external magnetic field with said AC electromagnet. 8.The method recited in claim 6, further comprising: providing a DCelectromagnet mounted to said hand-held frame; and generating saidexternal magnetic field with said DC electromagnet.
 9. The methodrecited in claim 5, further comprising: providing a permanent magnetmounted to said hand-held frame as said external magnetic field source;and generating an electromagnetic external field with said permanentmagnet.